The stream can contain the contents of a single font file, or a zip file containing a set of fonts. Obfuscated fonts are only supported within zip files.
Streams might be stored and retrieved from isolated storage, or could come from a stream in a StreamResourceInfo obtained from Application..::.GetResourceStream.
Using a FontSource to supply a value for TextBox..::.FontSource or TextBlock..::.FontSource is an alternative technique to specifying a URI portion for a FontFamily to reference a font file. This gives you more options for packaging or obtaining a font file. Basically you obtain a stream of either a font file or a zip of font files, and set FontSource using that stream. This makes that font file available to the TextBox or TextBlock object. You can then set Control..::.FontFamily or TextBlock..::.FontFamily with a simple fontFamilyName string to reference one of the specific families in the current FontSource value.